1. Field of the Invention
The present invention relates to an active matrix type flat panel display device and, more particularly, to a top-emitting organic light-emitting device (OLED) capable of reducing or preventing a voltage drop, with a cathode bus line and a cathode electrode being electrically coupled and method for fabricating the same.
2. Description of Background
In general, an organic light emitting device is an emissive display device which may be classified into a bottom-emitting structure type and a top-emitting structure type based on the direction of light emitted from an organic emission layer. The top-emitting organic light-emitting device emits light in a direction opposite to a substrate where pixels are arranged, and may increase an aperture ratio when compared to the bottom-emitting structure which emits light toward the substrate where the pixels are arranged.
The light is emitted toward the substrate for encapsulation in the top-emitting structure, so that a transparent electrode is used for a cathode electrode. A transparent conductive layer, such as ITO or IZO, is used for the transparent electrode. However, the transparent conductive layer may have a higher work function, so that it is difficult to use it for the cathode electrode.
To cope with this problem, a thin metal having a lower work function is deposited on an organic emission layer for the cathode electrode to form a semitransparent metal layer. A thick transparent conductive layer is then deposited on the semitransparent metal layer to form a transparent electrode having a stacked structure.
However, in the cathode electrode of the stacked structure, since the transparent conductive layer, such as ITO or IZO, is deposited after an organic thin-film layer is formed, a low temperature deposition process minimizes degradation of an electroluminescence (EL) layer due to heat or plasma. When the ITO or IZO is deposited at a lower temperature, film quality may become worse and specific resistance may become higher.
The cathode electrode is a common electrode, and the same voltage should be applied to all pixels arranged in a pixel portion. However, a voltage drop (namely, an IR drop) occurs due to the high specific resistance of the cathode electrode. This causes different voltage levels to be applied to the pixels in accordance with their arranged positions. Thus, when a cathode voltage is applied from an external terminal to the cathode electrode, pixels arranged near the external terminal and pixels spaced apart from the external terminal do not have the same voltage, which causes the voltage drop. This voltage difference per pixel position may cause non-uniformity of luminance and/or image quality.
In particular, the voltage drop problems may become more serious in a top-emitting organic light-emitting device of medium and large size. Korea Patent Application No. 2002-0057336 discloses a technique that uses a cathode bus line in the top-emitting structure. The cathode bus line is connected to an external terminal and contacts a cathode electrode, so that the cathode electrode is connected to the external terminal through the cathode bus line.
The method for connecting the cathode bus line to the cathode electrode may prevent the voltage drop of the cathode electrode with respect to the pixel position. However, when a carrier transporting layer, such as an organic layer, is formed on the entire surface of the substrate between the cathode bus line and the cathode electrode, the cathode bus line and the cathode electrode are not electrically coupled to with each other.